Light emitting diode (LED) provides many advantages over the traditional tungsten light bulb, such as longer lifespan, lower power consumption, higher luminosity and less fabrication materials. These days as energy saving and carbon reduction are growing concerns in the mind of many people, LED has been widely used. Apart from adopted on traffic lights or signal lights on electric appliances, it also gets growing acceptance in general houses. Techniques to couple the LED with the conventional light bulb have been proposed in prior art. For instance, R.O.C. patent No. I293807 entitled “LED light bulb equipped with a constant current circuit” discloses an LED light bulb which includes a lamp cap, a lamp shell, a plurality of LEDs coupled in series and a step-down constant current circuit. The lamp cap has an electrode connecting to a power source. The LEDs are connected to the step-down constant current circuit to provide constant current to allow the LEDs to emit light. The LED light bulb thus formed can be mounted onto a conventional socket and used directly.
However, the luminosity provided by a single LED is still inadequate. Hence the general LED light bulb usually contains multiple LEDs. As a result, the substrate holding the LEDs accumulates a great amount of waste heat. If the waste heat cannot be dissipated efficiently from the substrate to the outside, high temperature affects lighting efficiency of the LEDs, and the lifespan of the LEDs suffers. To address this issue, LED light bulbs equipped with metal heat sink have been developed, such as R.O.C. patent Nos. M345944, M358247 and M381743, and U.S. publication Nos. 2011/0068692 and 2009/0303736. They mainly provide a substrate to hold LEDs and a power receiving base electrically connected to a commercial power source. The substrate and power receiving base are interposed by an exposed heat sink which further holds a driving circuit inside. To improve cooling effect of the heat sink, the heat sink generally is made of metal and contains a plurality of radiation fins.
Aside from the aforesaid technique to dissipate the waste heat via the metal heat sink, another cooling approach also is proposed that transmits the waste heat from the substrate via the heat sink to the power receiving base, such as U.S. publication No. 2009/0052186 which discloses an LED light bulb including a lamp shell, a power receiving base, a heat sink located between the lamp shell and power receiving base, and a substrate. The heat sink has an upper portion coupled with an opening of the lamp shell. The heat sink has a protrusive ring in the middle portion and a thread section at the lower surface to couple with the power receiving base. The waste heat generated by the substrate is transmitted via the heat sink to the power receiving base for dissipating. The cooling structure for the LED thus formed in simpler and straightforward. However, it also has a drawback, i.e. coupling with the lamp shell is accomplished via the heat sink rather then the power receiving base. Hence the protrusive ring is exposed outside the power receiving base. As the LED light bulb has to pass a severe high voltage applying test under an external voltage of 4000 volts, electric power could pass through the protrusive ring and break through insulation of the insulator to damage the conversion circuit in the heat sink and make the LED light bulb fail to pass the safety regulation test. The LED light bulb thus formed has the concern of current leakage through the heat sink when in use.
On the other hand, aluminum has a heat conductivity coefficient up to 237 Wm−1K−1 and can be abundantly supplied. Compared with other metals, it also has a higher thermal conductivity and is cheaper. Hence the conventional LED light bulbs generally adopt an aluminum substrate laid with conduction circuits to drive the LEDs to emit light. But most conventional LED light bulbs have the conduction circuits formed merely on one side of the aluminum substrate to allow the aluminum substrate to project light via one surface. To enhance the luminosity of the LED light bulb, one side of the aluminum substrate where the conduction circuits are laid is clustered with a greater number of LEDs. As a result, a greater amount of waste heat is generated and creates a concern of damaging the LEDs.